The present invention relates to a carbonaceous or graphitic carbon material which is obtained from a mesophase pitch as a raw material and which is excellent in heat resistance and chemical stability and has a uniform open cell structure and production processes of theses.
Various foam materials, such as a plastic foam, obtained from a resin as a raw material are widely applied to heat insulating materials, cushioning materials or the like. In recent years, it is thought to apply foam materials to fields requiring various properties not heretofore required such as heat resistance, thermal conductivity, chemical stability, electrical conductivity, strength, gas diffusivity, etc. For example, the above fields include a gas diffusion electrode of a fuel cell, a bipolar plate and the like. However, a conventional foam made of a thermoplastic resin can not be used. Accordingly, the use of a thermosetting resin foam, a carbon foam obtained by carbonizing a thermosetting resin foam or a foam made of a ceramic is discussed. Since, however, thermosetting resin-based foams form a hard-to-graphitize carbon, the thermosetting resin-based foams are poor in oxidation resistance at high temperatures or corrosion resistance in a chemical reaction and are insufficient in thermal conductivity. Further, foams made of a ceramic are excellent in oxidation resistance but are very poor in thermal conductivity and electrical conductivity.
As a means for providing materials having possibility to satisfy these requirements, one of the present inventors has newly found that a novel foam material having excellent properties such as high chemical stability, heat resistance, oxidation resistance, etc., can be produced by controlling the density of a mesophase pitch by means of a heat-treatment of the mesophase pitch under the pressurization of an inert gas, and has announced this finding (Preparation, structure and application of mesophase pitches prepared from aromatic hydrocarbons using HF/BF3 as catalystsxe2x80x9d TANSO 1992 [155] 370-378, I. Mochida, Y. Korai, K. Shimizu, S-H. Yoon, R. Fujiura.).
The above paper describes that, since the density and pore size of a mesophase pitch can be controlled according to a pressure or a temperature-increasing rate in a foaming step, no curing and no foaming agent are required and that a graphite foam can be produced by heat-treating this carbonaceous foam.
Further, U.S. Pat. No. 6,033,506 also discloses a process of producing carbon foam wherein a pitch is heat-treated under the application of a pressure of up to 1,000 psi (approximately 6.8 MPa) with an inert gas to produce a carbon foam.
As described above, it has been found that a foam material can be produced by heat-treating a pitch such as a mesophase pitch under the pressurization of an inert gas. However, since the nature of a raw material pitch required for industrially stably producing a foam of which the shape and size of a cell are uniformly controlled and which has excellent properties such as chemical stability, heat resistance and oxidation resistance, is unknown, a pitch designed as a raw material for a foam has been not yet provided. Therefore, it is difficult to control the physical properties of a foam. Further, nonuniform parts exist in large quantities in a generated foam during the production of a foam so that the problem is that the yield of a product decreases as a result.
That is, it is difficult to industrially stably produce a foam which has a uniform cell structure and various properties not hitherto attained, such as heat resistance, thermal conductivity, chemical stability, electrical conductivity, strength and diffusivity, from a generally-used conventional pitch with the physical properties of the foam having the above properties being controlled.
For carrying out a heat-treatment at a high temperature of 500xc2x0 C. or higher and under a high-pressure condition of, for example, at least 6 MPa, a special reactor which can endure a high temperature and a high pressure is required. Therefore, it is difficult to produce a foam industrially economically. Accordingly, it is required to control the nature of a carbon foam such as a bulk density under a low-pressure condition.
It is an object of the present invention to provide a pitch from which a foam having a uniform cell structure and various properties not hitherto attained, such as heat resistance, thermal conductivity, chemical stability, electrical conductivity, strength, diffusivity, etc., can be industrially stably produced with the physical properties of the foam being controlled and provide a carbon foam and a graphite foam satisfying the above properties from the pitch.
It is another object of the present invention to provide a carbon foam and a graphite foam which are obtained by controlling the nature of a carbon foam having a uniform cell structure and properties such as heat resistance, thermal conductivity, chemical stability, electrical conductivity, strength, gas diffusivity, etc., under a low-pressure production condition and the production processes of these.
According to the present invention 1, there is provided a carbon foam which is obtained by heat-treating a mesophase pitch whose softening point is 300xc2x0 C. or less according to an elevated flow tester, whose ratio (Daromatic/Daliphatic) of the absorption intensity of an aromatic Cxe2x80x94H stretching vibration, measured with FT-IR, to the absorption intensity of an aliphatic Cxe2x80x94H stretching vibration, measured with FT-IR, is 4.0 or less and whose optically anisotropic content is at least 80%.
According to the present invention 1, further, there is provided a carbon foam as recited above, wherein the mesophase pitch has an aromatic carbon index fa value of from 0.80 to 0.97,
the aromatic carbon index fa value being represented by the formula (1),
fa=1xe2x88x92(H/C)/x(1+(Daromatic/Daliphatic)xc3x97(xcex5aliphatic/xcex5aromatic))xe2x80x83xe2x80x83(1) 
in which (H/C) is an atom ratio of hydrogen to carbon in the pitch, (Daromatic/Daliphatic) is a ratio of the absorption intensity of an aromatic Cxe2x80x94H stretching vibration, measured with FT-IR, to the absorption intensity of an aliphatic Cxe2x80x94H stretching vibration, measured with FT-IR, x is an average number of hydrogen bonded to carbon other than aromatic carbon (x=2), and xcex5aliphatic/xcex5aromatic is a specific extinction (=2).
According to the present invention 1, further, there is provided a carbon foam as recited above, wherein the mesophase pitch is a pitch obtained by polymerizing a fused polycyclic hydrocarbon or a substance containing a fused polycyclic hydrocarbon in the presence of hydrogen fluoride and boron trifluoride.
According to the present invention 1, further, there is provided a carbon foam as recited above, which is obtained by heat-treating the above mesophase pitch at a temperature of from 400xc2x0 C. to 800xc2x0 C. under the application of pressure of 0.1 MPa or more with an inert gas.
According to the present invention 1, further, there is provided a carbon foam which is obtained by further heat-treating the carbon foam obtained by the above heat-treatment at a temperature of from 600xc2x0 C. to less than 2,000xc2x0 C.
According to the present invention 1, further, there is provided a carbon foam as recited above, which has a bulk density of from 0.20 g/cm3 to 0.65 g/cm3 and has a density, measured using helium as a substitution medium, of from 1.3 g/cm3 to 1.5 g/cm3.
According to the present invention 1, further, there is provided a carbon foam as recited above, which has a porosity p of from 50% to 90% and an open cell rate of from 90% to 100%,
the porosity p being represented by the formula (2),
P=(1xe2x88x92d/Dr)xc3x97100xe2x80x83xe2x80x83(2) 
in which d is a bulk density and Dr is a density measured using helium as a substitution medium after the foam is pulverized to 150 microns or less,
the open cell rate being represented by the formula (3),
xcfx86xe2x88x92D/Drxc3x97100xe2x80x83xe2x80x83(3) 
in which D is a density measured using helium as a substitution medium and Dr has the same meaning as recited above.
According to the present invention 1, further, there is provided a graphite foam which is obtained by heat-treating the carbon foam recited above at a temperature of 2,000xc2x0 C. or higher.
According to the present invention 1, further, there is provided a graphite foam as recited above, which has a bulk density of from 0.3 g/cm3 to 1.0 g/cm3 and has a density, measured using helium as a substitution medium, of 2.0 g/cm3 or more.
According to the present invention 1, further, there is provided a graphite foam as recited above, which has a porosity of from 50% to 90% and an open cell rate of from 90% to 100%.
According to the present invention 2, there is provided a carbon foam which is obtained by heat-treating, at a temperature of from 400xc2x0 C. to 800xc2x0 C. under the application of pressure of from 0.1 MPa to 5 MPa, preferably from 0.1 MPa to 4 MPa, more preferably from 0.1 MPa to 3 MPa, with an inert gas, at least one mesophase pitch selected from the group consisting of the following mesophase pitches a, b, c and d which are obtained by polymerizing a fused polycyclic hydrocarbon or a substance containing a fused polycyclic hydrocarbon in the presence of hydrogen fluoride and boron trifluoride as catalysts,
a. a mesophase pitch having an aromatic carbon index fa value of from 0.90 to 0.97, which mesophase pitch is obtained by removing the catalysts after the polymerization and then heat-treating the resultant pitch at a maximum treatment temperature in the range of from 300xc2x0 C. to 500xc2x0 C.,
the aromatic carbon index fa being determined by the following formula (1),
fa=1xe2x88x92(H/C)/x(1+(Daromatic/Daliphatic)xc3x97(xcex5aliphatic/xcex5aromatic))xe2x80x83xe2x80x83(1) 
in which (H/C) is an atom ratio of hydrogen to carbon in the pitch, x is an average number of hydrogen bonded to carbon other than aromatic carbon (x=2), (Daromatic/Daliphatic) is a ratio of the absorption intensity (Daromatic) of an aromatic Cxe2x80x94H stretching vibration, measured with FT-IR, to the absorption intensity (Daliphatic) of an aliphatic Cxe2x80x94H stretching vibration, measured with FT-IR, and xcex5aliphatic/xcex5aromatic is a specific extinction (=2),
b. a mesophase pitch which is obtained by carrying out the polymerization in a catalyst molar ratio (fused polycyclic hydrocarbon/hydrogen fluoride/boron trifluoride) of hydrogen fluoride/fused polycyclic hydrocarbon=0.2xcx9c1.0 and boron trifluoride/fused polycyclic hydrocarbon=0.01xcx9c0.1 at a reaction temperature of from 100 to 350xc2x0 C., removing the catalysts and then heat-treating the resultant pitch at a maximum treatment temperature in the range of from 300xc2x0 C. to 500xc2x0 C.,
c. a mesophase pitch having a fa value of from 0.90 to 0.97 and a softening point, determined with a flow tester, of 300xc2x0 C. or less, which mesophase pitch is obtained by removing the catalysts from the mesophase pitch obtained by polymerizing in the same catalyst molar ratio as that in the above b and at the same reaction temperature as that in the above b and then heat-treating the resultant pitch at a maximum treatment temperature of from 400xc2x0 C. to 500xc2x0 C., and
d. a mesophase pitch having a softening point of 250xc2x0 C. or higher according to a flow tester, which mesophase pitch is obtained as a residue when at least one mesophase pitch obtained from the group consisting of the above a, b and c is extracted with a solvent.
According to the present invention 2, further, there is provided a process for the production of a carbon foam, comprising heat-treating at least one mesophase pitch selected from the group consisting of the above a, b, c and d at a temperature of from 400xc2x0 C. to 800xc2x0 C. under the application of pressure of from 0.1 MPa to 5 MPa, preferably from 0.1 MPa to 4 MPa, more preferably from 0.1 MPa to 3 MPa, with an inert gas.
According to the present invention 2, further, there is provided a process for the production of a carbon foam, comprising pulverizing at least one mesophase pitch selected from the group consisting of the above a, b, c and d, molding the pulverized mesophase pitch at a room temperature or under heat and then heat-treating the molded mesophase pitch at a temperature of from 400xc2x0 C. to 800xc2x0 C. under the application of pressure of from 0.1 MPa to 5 MPa, preferably from 0.1 MPa to 4 MPa, more preferably from 0.1 MPa to 3 MPa, with an inert gas.
According to the present invention 2, further, there is provided a carbon foam which is obtained by further heat-treating the carbon foam obtained from at least one mesophase pitch selected from the group consisting of the above a, b, c and d at a temperature of from 600xc2x0 C. to less than 2,000xc2x0 C. Owing to this treatment, a density, electrical physical properties and thermally physical properties can be controlled in proper ranges suited for an intended use.
According to the present invention 2, further, there is provided a process for the production of a carbon foam, which process comprises further heat-treating the above carbon foam at a temperature of from 600xc2x0 C. to less than 2,000xc2x0 C.
According to the present invention 2, further, there is provided a graphite foam having a bulk density of 0.30 g/cm3 or more, which is obtained by heat-treating the above carbon foam at a temperature of 2,000xc2x0 C. or higher.
According to the present invention 2, further, there is provided a process for the production of a graphite foam, which process comprises heat-treating the above carbon foam at a temperature of 2,000xc2x0 C. or higher.